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This is Hacker Public Radio episode 3,781 from Monday the 30th of January 2023.

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Today's show is entitled, The Jewel Thief.

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It is the 20th show of Andrew Conway and is about 13 minutes long.

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It carries a clean flag.

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The summary is, using the Joel Thief to suck energy out of flat batteries.

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Hello Hacker Public Radio people, this is McNallow also known as Andrew and today I'm

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going to tell you about a delightful little circuit called The Jewel Thief.

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The Joel Thief allows you to extract energy from an otherwise flat battery and more remarkably

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than that, using a flat battery, so I'll say I'm using a AA battery, it's was 1.5 volts

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when it started out its life, it's now down at around about 1 volt and even at 1 volt the

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Joel Thief's circuit is more than capable of allowing you to light up a two volt LED or need

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a string of LEDs of even higher voltage than that.

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So the magic of this circuit is that it's so simple as well as being able to use a flat battery

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to power an LED that requires a higher voltage and the secret of the circuit is very fast switching.

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So the LED isn't in fact on all the time, but it's on enough of the time that you can't really

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tell that it's flickering on and off. In fact, the circuit I've built as far as I can tell,

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it's flickering on and off 100 kHz, which is 100 000 times a second and I'm not going to pick

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that up with my eye certainly. So it only uses for components, the battery and for components,

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so I have a flat battery, and it's worth noting that when you I say a battery is flat,

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you will not be able to use it to power most things, but of course flat is relative.

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When a battery is exhausted, it's exhausted for the particular task that you put it to, for example,

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lighting up an LED torch, running a radio, running a Bluetooth mouth, Bluetooth mouse,

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I've got that right in the end. Once the voltage goes too low, without some degree pro-curry trickery,

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you can't use it anymore. But if what you really just want to do is light an LED,

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there is a way to coax remaining energy on the battery. Indeed,

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something like 50% of the energy in the battery is still there, it's just coming out to a low voltage,

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so if you can, put it to another use, that would be great. So let me just describe the circuit.

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So the four components are a resistor. The one I've got in front of me is a 10 kWh resistor,

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but I used a 1 kWh resistor to start with, and that would work too. I've got a transistor and

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NPN transistor, it's a BC-548, which is a totally bog-standard transistor, nothing fancy about

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that at all. A yellow LED, I guess, is a work with almost any color of LED, and the fourth,

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and most exciting component, is myself, one, and doctor. Now this is not an ordinary inductor.

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An ordinary inductor is a set, which is just a coil of wire around a ferromagnetic core, usually.

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This has a ferromagnetic core, it's in the shape of a torus, and most doctors will look like that.

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But the clever thing about this inductor is it's got two windings. One that goes one way around the

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torus, and another that goes the opposite way around the torus, and that's the key to how this

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thing works. It's actually quite easy, a little fiddly, to widen one of these yourself. So what I did,

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I had a dimmer switch for my kitchen light, it was a main voltage dimmer switch,

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that eventually looked up in through some LED driver circuits to all the lights in my kitchen.

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The switch on it became soft, so I had to replace it. But I don't like to throw things out,

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so I kept it thinking, let me keep him in handy one day, and indeed it did, because when I opened

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it up, I found it had an inductor. So I dismantled the inductor. Now a normal inductor just has,

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as I say, one winding of wire around the ferrite core. This is a ferrite bead, so it's a quite large

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actually, it's about centimeter across, and maybe a half a centimeter deep, and it's like a

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donut of some ferromagnetic material. So I removed just unwound by hand, so I'll start watching

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some television program, look to me a few minutes to unwind, about a meter worth of the wire

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that was well done to the inductor. Then I took this wire and folded it over, so I have two

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free ends, and the other end is now a fold, where the wire is folded in half. I anchored the two

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free ends by wrapping it around the ferrite bead once, and then proceeded to thread through

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the looped end, the folded end, repeatedly until I had windings that went all the way around

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the torus. Let's say, done that, I cut the wire at the fold, so I now have four

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ends, and using a multimeter, I actually, the next thing I did was I used a bit sandpaper

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to scrape off the enamel, because it's all, the wire is insulated, it would work as an inductor,

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if it was just bare copper wire, it's not, it's enameled wire. So at the ends, the four ends

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I had, I used sandpaper to scrape off that enamel, so I could make a electrical contact, and then

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what I did was I just joined two bits of wire that weren't connected to each other, and I used

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a multimeter to figure out which two ends didn't really matter, it doesn't matter which two ends,

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as long as they're not showing continuity in a multimeter, so I took those two ends and joined them

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together, and that will be the north north, the positive end of my, the inductor I'll use in the

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circuit. Now it's not an ordinary inductor, so it's got a common positive, and it's got two

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negative ends, if you like, and so what you do is you take the positive end and you connect that to

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your flat battery, so the positive terminal of your flat battery, one of the two negative ends,

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one of them gets connected to a resistor, and then that resistor gets connected to the base of the

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transistor, so the base of the transistor, if you imagine it, is like the tap that you can turn,

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so you can turn the transistor on and off, which will be crucial in a moment, so the base of the

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transistor controls where the current can flow through the rest of the transistor. The other

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negative wire that comes out in the inductor gets connected to the collector of the transistor,

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and the third and final leg of the transistor, the emitter gets connected to the negative terminal

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of the battery, and you also then take your LED and connect the positive leg of the LED to the

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collector, which is also connected to one of the ends of the negative end of the inductor, and

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that's the positive, so it's the positive end of the LED, negative leg of the LED gets connected

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to the emitter, and to zero volts, or the negative terminal of the battery. Look at the circuit diagram,

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it'll be easier than that, but this is the crucial bit, so what happens is when you connect all this

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up, current will flow from this flat battery, and it only needs a small amount, one volt is plenty,

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through a resistor, one kilotone tank alone doesn't matter, as tiny little current will flow

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into the base of the transistor, and then the transistor after a short delay will turn on.

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When the transistor turns on, it will allow current to flow through the other winding, so one winding

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of the inductor, remember, goes to the base of the transistor to open it up, the other winding

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comes down through the to the collector, and at first it can't the current can't flow that path,

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because the transistor is off, but once some current flows into the base through the other winding

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in the inductor, then the current can flow through the transistor, and it barely very low resistance,

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so very quickly, a much larger current will start to flow through the transistor, and a much larger

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current will start to flow through the other winding of the inductor, and because the two winding

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is the inductor, shear the same core, and they're winding opposite directions, what this does

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is it stops the current flowing into the base of the transistor, so what happens? Well,

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when that current stops, the transistor shuts off the tap, shuts, and then that main current

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path suddenly closes off, and this is the key property of an inductor, if you rapidly change

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the current flowing through an inductor, you get a spike in voltage, so the voltage, the

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inductor equation is voltage equals inductance times rate of change of current with time, so if you

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vary suddenly switch off the current, you'll get a spike in voltage, and so even when you're only

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got one say a one volt battery, you can easily produce two volts, three volts, four volts are

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potentially very large voltage is actually for a brief time, and that allows you to light

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the LED, which is connected across the collector no matter of the transistor.

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And as I said at the beginning, this happens extremely quickly. All of that takes place,

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say 100,000 times a second, depends on the value of the inductor, I've got quite a high value of

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inductor just because of what came out of the dimmer switch, and to a lesser extent it depends

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on the resistor, but really the switching as far as I understand it is mostly depend on the transistor,

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and the timings inside the transistor. Now the transistor that I'm using is not really designed

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for switching, and if you look at status sheet, it doesn't tell you anything about the timings,

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other transistors, MOSFETs, they do tell you stuff about timings, but this PC 548, no,

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you don't get any information, because it's not really intended for that, it doesn't matter,

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it just works. Now I have tried to understand the equations that governness and it's for such

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a simple circuit with four components, it's deceptively simple, because the equations are actually quite

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complex, so I actually went right back to Maxwell's equations, if you've done any physics you

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might have encountered Maxwell's equations, and I was able to derive the inductor equation in this case,

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and for those of you that are interested, it looks very much like the usual inductor equation,

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so the voltage drop across the inductor is equal to the inductor times the rate of change

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of the difference of the two currents that are flowing in the two's winding to the inductor.

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But actually to try and model what's going on in the circuit using that equation is difficult,

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because as I said earlier, it isn't the inductor in the resistor that is where the magic is

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happening, well some of the magic's happening in the inductor, a lot of the magic is happening in the

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transistor, and transistors are nonlinear devices, and if you don't have documentation and the

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time delays involved, I don't see really how you can model the joule thief in any simple way,

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you can certainly model it, but it's not simple, but it doesn't really matter, because if you want to

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just light up an LED with a flat battery, well a joule thief is certainly the way to go.

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Now if you would like to build your own, I highly recommend about half and half our video

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by Clive Mitchell or Big Clive, as he's known in YouTube, he is the person who coined the term

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joule thief, which is a lovely little pun, but the circuit itself is much older, it

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I think maybe the first patterns are almost 90 years ago, but those components are very different

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from the ones we would use today, and the ones I've got in front of me, and I think Clive references

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a letter in the magazine that was published in 1999, but anyway, for those details and for a lovely

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clear description and how to make it and how it works as well, do refer to Big Clive's video

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which is a link I'll put in the short notes. I hope you've found that, uh, interesting. Bye bye.

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You have been listening to Hecker Public Radio at Hecker Public Radio.org.

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